The present invention relates to current sources and, more particularly, to a circuit and method for providing an output current having a regulated and known magnitude.
Current sources comprising current mirrors for providing output currents that are a function of a reference current are well known in the art. An example of a prior art circuit for providing an output current that is proportional to a reference current is described in U.S. Pat. No. 4,642,551. This current source comprises a current mirror formed by two transistors having their bases coupled together and their collectors respectively coupled to the inverting and non-inverting input of an operational amplifier. A reference current is applied to the collector of the transistor having its collector connected to the non-inverting input of the operational such that an output current is produced through the collector-emitter conduction path of the other transistor of the current mirror which is proportional to the reference current. The operational amplifier provides feedback current to the current mirror which varies as a function of the output current whereby the output current tends to be held constant, i.e., is regulated.
Typically, the output current from the above described current source is turned around using a conventional PNP current mirror coupled to the collector of the output thereof in order to source current to a load. Although this type of regulated current source functions quite well in bipolar circuits fabricated using standard bipolar integrated processes, its performance will suffer if fabricated in integrated circuit form using contemporary low voltage bipolar integrated circuit processes. This problem arises due to the low forward current gain, beta, of PNP transistors associated with the low voltage processes currently used in the industry. Because of the low beta of PNP transistors the prior art suffers loading affects which is highly undesirable.
Hence, a need exists for providing a regulated current source that is suited to be manufactured in integrated circuit form using conventional low power, low voltage bipolar integrated circuit processes.